Precision-casting core, precision-casting core manufacturing method, and precision-casting mold

ABSTRACT

A coating layer including solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide is formed on a surface of a sintered precision-casting core body mainly including silica particles so as to seal holes formed in the surface. As a result, it is possible to prevent the breakage of the core during casting.

FIELD

The present invention relates to a precision-casting core, a precision-casting core manufacturing method, and a precision-casting mold.

BACKGROUND

As a precision-cast product, for example, a turbine blade used in a gas turbine is known. In the gas turbine, a working fluid is heated by a burner so as to be a high-temperature/high-pressure working fluid, and the turbine is rotated by the working fluid. That is, the working fluid compressed by a compressor is heated by the burner so as to increase the energy of the working fluid, the energy is recovered by the turbine so as to generate a rotation force, and hence electric power is generated by the rotation force. The turbine is provided with a turbine rotor, and the outer periphery of the turbine rotor is provided with at least one gas turbine blade.

Here, the gas turbine blade is exposed to a high temperature. As a countermeasure, a cooling medium flows in the gas turbine blade so as to cool the gas turbine blade. For this purpose, the gas turbine blade is provided with an internal cooling structure. Then, in order to form the internal cooling structure, a core having the same shape as a cooling medium flow passage is disposed and the core is removed after casting. The core is removed while being dissolved in an alkali (for example, NaOH or KOH) solution. As a result, for example, the internal cooling structure for the turbine blade is formed.

As the core, a ceramic core using ceramic particles has been used from the past (Patent Literature 1).

Here, a precision-casting core can be obtained by molding a silica material such as melted silica (SiO₂) through, injection molding or slip casting and performing a heat treatment thereon.

The injection molding method, is a method of obtaining a compact by kneading ceramic powder and wax, injecting a material obtained by heating and melting the wax into a metal mold, and cooling and hardening the material.

Further, the slip casting method is a method of preparing slurry by mixing ceramic powder with water or the like, pouring the slurry into a mold formed of a material such as gypsym absorbing a solution, and drying the slurry so as to obtain a desired molded shape.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Laid-open Patent Publication 6-340467

SUMMARY Technical Problem

Incidentally, since the existing core is mainly manufactured in consideration of alkali solubility, a problem arises in that the high-temperature strength of the core is low. Further, in the injection molding method, a plurality of holes is formed in the surface of the core which is sintered after molding. As a result, a problem arises in that, the strength is low and the core may be broken from the holes as the start points during casing.

Accordingly, there has been a demand for the precision-casting core the high-temperature strength of which is improved.

The invention is made in view of the above-described circumstance, and an object thereof is to provide a precision-casting core with improved high-temperature strength, a precision-casting core manufacturing method, and a precision-casting mold.

Solution to Problem

According to a first aspect of the present invention to solve the above mentioned problems, there is provided a precision-casting core obtained by forming an alkoxide coating layer including an alkoxide material on a surface of a sintered precision-casting core body mainly including silica particles.

According to a second aspect, there is provided a precision-casting core obtained by forming an alkoxide-silica fume coating layer including an alkoxide material and silica fume on a surface of a sintered precision-casting core body mainly including silica, particles.

According to a third aspect, in the first or second aspects, there is provided, the precision-casting core, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum-alkoxide.

According to a fourth aspect, there is provided a precision-casting mold used to manufacture cast metal, comprising the precision-casting core of the first or second aspects having a shape corresponding to a cavity inside the cast metal and an outer mold corresponding to the shape of the outer peripheral surface of the cast metal.

According to a fifth aspect, there is provided a precision-casting core manufacturing method comprising immersing a sintered body of a precision-casting core body mainly including silica particles into an alkoxide material, drying the sintered body and heating the sintered body so as to form a coating layer on the surface of the precision-casting core body.

According to a sixth aspect, there is provided a precision-casting core manufacturing method comprising immersing a sintered body of a precision-casting core body mainly including silica particles into an alkoxide-silica fume material of an alkoxide material and silica fume, drying the sintered body and heating the sintered body so as to terra a coating layer on the surface of the precision-casting core body.

According to a seventh aspect, in the fifth or sixth aspects, there is provided the precision-casting core manufacturing method, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.

According to an eighth aspect, in the third aspect, silicon ethoxide or silicon butoxide is used as the silicon alkoxide.

Advantageous Effects of Invention

Since the invention has a configuration in which the coating layer of the alkoxide material is formed on the surface of the sintered precision-casting core body, the holes formed in the surface during sintering are sealed. Accordingly, there is an effect, that the breakage of the core is prevented during casting in that the strength of the core is improved and the holes are sealed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional configuration diagram of a precision-casting core.

FIG. 2 is a flowchart illustrating an example of a process of a casting method.

FIG. 3 is a flowchart illustrating an example of a process of a mold manufacturing method.

FIG. 4 is a schematic diagram illustrating a core manufacturing process.

FIG. 5 is a schematic perspective view illustrating a part of a metal mold.

FIG. 6 is a schematic diagram illustrating a wax pattern manufacturing process.

FIG. 7 is a schematic diagram illustrating a configuration of applying slurry to a wax pattern.

FIG. 8 is a schematic diagram illustrating an outer mold manufacturing process.

FIG. 9 is a schematic diagram illustrating a part of a process of the mold manufacturing method.

FIG. 10 is a schematic diagram illustrating a part of a process of the casting method.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the invention will be described in detail with reference to the drawings. Furthermore, the invention is not limited to the description below. Further, the components to be described below include a component which can be easily supposed by the person skilled in the art, a component which has substantially the same configuration, and a so-called equivalent component.

First Embodiment

FIG. 1 is a cross-sectional configuration diagram of a precision-casting core.

A precision-casting core according to the invention is obtained by forming a coating layer of two kinds of silica materials having different particle diameters on a surface of a sintered precision-casting core body (hereinafter, referred to as a “core body”) mainly including silica particles.

As illustrated in the upper stage of the cross-sectional view of the core body as the sintered body of FIG. 1, a plurality of holes 18 c is formed in a surface 18 b of a core body 18 a during sintering.

In the invention, as illustrated in the lower stags of FIG. 1, the doles 18 c are sealed by coating the holes 18 c formed in the surface by a coating layer 19 a.

Here, the core body 18 a mainly includes silica particles, for example, melted silica (SiO₂) such as silica sand and silica flour.

The core body 18 a is manufactured by a known method in which wax is added to a mixture prepared by mixing silica particles, for example, silica flour (for example, 800 mesh (10 to 20 μm)) and silica sand (for example, 220 mesh (20 to 70 μm)) at the weight ratio of 1:1 and is heated and kneaded so as to obtain a compound.

A core compact is obtained by injection molding the obtained compound.

Subsequently, the core body 18 a is obtained by performing a degreasing treatment to, for example, 600° C. and a sintering treatment at, for example, 1,200° C.

In the invention, the coating layer 19 a is formed on the surface 18 b of the core body 18 a as the obtained sintered body.

The alkoxide material is used in the coating layer 19 a.

Here, the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.

Silicon ethoxide or silicon butoxide is used as silicon alkoxide, and ethanol or butanol is used as solvent.

Further, when two kinds of alkoxide are mixed, a mixed alkoxide material obtained by mixing silicon alkoxide and aluminum alkoxide is used, and for example, alcohol solvent such as butanol is used as solvent.

When mixed alkoxide is prepared, mixed alkoxide of silicon ethoxide and aluminum isopropoxide is dissolved in a butanol solution.

Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) is mixed at the molar ratio of 2:3 so as to prepare organic raised alkoxide.

The core sample is immersed into prepared solely alkoxide or mixed alkoxide and is pulled up so as to form a silicon layer or a silicon-aluminum alkoxide layer on the surface 18 b of the core body 18 a and to precipitate a silicon, component or silicon-aluminum, alkoxide component even in the hole 18 c of the core surface thereof.

Since solely alkoxide or mixed alkoxide is dissolved in an alcohol solution daring immersing, solely alkoxide or mixed alkoxide easily penetrates into the core body, and hence the good coating layer 19 a is formed thereon.

Subsequently after the drying process, a heat treatment is performed, at, for example, 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided with the coating layer 19 a.

In the heat treatment, in the case of mixed, alkoxide, the silicon-aluminum alkoxide layer changes to inorganic mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Thus, it is possible to obtain the core 18 in which the core body 18 a is covered by the mullite coating layer 19 a.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, a high casting temperature can be handled.

In this way, according to the invention, since the plurality of holes formed in the surface is sealed, it is possible to prevent a problem in which the core is broken during casting from the holes as the start points in the related art. Accordingly, the high-temperature strength of the precision-casting: core is improved.

Test Example 1

Hereinafter, a test example for verifying the effect of the invention will be described.

In the test example, a compound was first obtained by adding wax to a mixture prepared by mixing silica flour (800 mesh) and silica sand (220 mesh) at the weight, ratio of 1:1 and heating and kneading the mixture. Here, “MCF-200C” (product name) manufactured by Tatsumori Ltd. was used as silica flour, “RD-120” (product name) manufactured by Tatsumori Ltd. was used as silica sand, and “Cerita Wax F30-75” (product name) manufactured by Paramelt Co., Ltd., was used as wax.

A compact was obtained by injection molding the obtained compound.

As a test sample, a sample having a width of 30 mm, a length of 200 mm, and a thickness of 5 mm was obtained.

Next, a sample for a core body was obtained by performing a decreasing treatment to 600° C. and a sintering treatment at 1,200° C.

Coating Layer 1

Next, silicon ethoxide was dissolved in an ethanol solution. The sample for the core body was immersed into the obtained silicon ethoxide and was pulled tip so as to form the coating layer 19 a of alkoxide on the surface thereof. Subsequently after the drying process, a heat treatment was performed at 1,000° C. so as to form the coating layer 19 a of inorganic silica including silicon ethoxide on the core body surface 18 b (Sample 1).

Coating Layer 2

Next, mixed alkoxide of silicon ethoxide and aluminum isopropoxide was dissolved in a butanol solution. Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) was mixed at the molar ratio of 2:3 so as to prepare organic mixed alkoxide.

The sample for the core body was immersed into the obtained mixed, alkoxide and was pulled up so as to form the coating layer 19 a of mixed alkoxide on the surface thereof. Subsequently after the drying process, a heat treatment was performed at 1,000° C. so as to form the coating layer 19 a on the core body surface 18 b by the mullite obtained by the reaction of mixed alkoxide of silicon ethoxide and aluminum isopropoxide (Sample 2).

As a comparative example, a core body without a coating layer was prepared as a comparative sample.

The strength of each of the test samples was measured.

Here, the strength test was performed based on “Bending Strength of Ceramics (1981)” of JIS R 1601.

The strength of the comparative sample without the coating layer of the conventional method was 20 MPs, but, to the contrary, the strength of Sample 1 of the silica coating layer of the coating layer 1 for the core body according to she method of the invention was 22 MPa. As a result, in the sample for the core body of the invention, it was acknowledged that the strength was improved by 10%.

Further, the strength of Sample 2 of the silica coating layer of the coating layer 2 for the core body according to the method of the invention was 24 MPa. As a result, in the sample for the core body of the invention, it was acknowledged that the strength was improved by 20%.

According to Sample 2 of the invention, since the high-temperature durability of the core is improved due to the mullite, it is possible to obtain a mold which is not deformed even, when the mold is held at a high temperature (for example, 1,550° C.) for a long period of time, for example, when a unidirectional solidified turbine blade is manufactured.

Hereinafter, a casting method using a mold having the precision-casting core of the invention disposed therein will be described.

FIG. 2 is a flowchart illustrating an example of a process of the casting method. Hereinafter, the casting method will be described with reference to FIG. 2. Here, the process illustrated in FIG. 2 may be totally automatically performed or may be performed by the operator operating each device dedicated for the process. In the casting method of the embodiment, a mold is manufactured (step S1). The mold may be manufactured in advance or may be manufactured for each casting.

Hereinafter, the mold manufacturing method of the embodiment performed by the process of step S1 will be described with reference to FIGS. 3 to 9. FIG. 3 is a flowchart illustrating an example of the process of the mold manufacturing method. Here, the process illustrated in FIG. 3 may be totally automatically performed or may be performed by the operator operating each device dedicated for the process.

In the mold manufacturing method, a core is manufactured (step S12). The core has a shape corresponding to the cavity inside the cast metal manufactured by the mold. That is, since the core is disposed at a portion corresponding to the cavity inside the cast metal, it is possible to suppress metal as the cast metal from flowing thereinto during casting. Hereinafter, the core manufacturing process will be described with reference to FIG. 4.

FIG. 4 is a schematic diagram illustrating the core manufacturing process. In the mold manufacturing method, a metal mold 12 is prepared as illustrated in FIG. 4(step S101). The metal mold 12 is formed so that an area corresponding to the core is formed as a cavity. A portion formed as a cavity of the core is formed as a convex portion 12 a. Furthermore, although it is illustrated in the cross-section of the metal mold 12 in FIG. 4, the metal mold 12 is formed so that an area other than an opening for injecting a material into a space therethrough or a hole for releasing air therethrough is basically formed as a cavity covering the entire periphery of an area corresponding to the core. In the mold casting method, ceramic slurry 16 is injected into the metal mold 12 from the opening for injecting a material, into the space of the metal mold 12 as indicated by an arrow 14. Specifically, a core 18 is manufactured by so-called injection molding while injecting the ceramic slurry 16 into the metal mold 12. In the mold manufacturing method, when the core 18 is manufactured inside the metal mold 12, the core 18 is separated from the metal mold 12 and the separated core 18 is baked while being disposed in a combustion furnace 20, Thus, the core 18 formed of ceramic is baked and hardened (step S102). Here, an “alkoxide material” is used as the material of the ceramic slurry 16.

Subsequently, in order to form a coating layer on the surface of the core 18, the sintered core 18 is immersed into a storage portion 17 filled with slurry 19 and is extracted so as to be dried (step S103). Subsequently, the immersed core 18 is extracted and is baked while being disposed in the combustion furnace 20. Thus, the coating layer 19 a is formed on the surface of the core 18 formed of ceramic (step S104).

In the mold casting method, the core 18 provided with the coating layer 19 a is manufactured as described above. Furthermore, the core 18 is formed of a material which is can be removed after the cast metal is hardened by a core removal process such as a chemical treatment.

In the mold manufacturing method, an outer metal mold is manufactured, after the core 18 is manufactured (step S14). The outer metal mold is formed in a shape in which the inner peripheral surface corresponds to the outer peripheral surface of the cast metal. The metal mold may be formed of metal or ceramic. FIG. 5 is a schematic perspective view illustrating a part of the metal mold. In a metal mold 22 a illustrated in FIG. 5, a recess formed in the inner peripheral surface corresponds to the outer peripheral surface of the cast metal. Furthermore, only the metal, mold 22 a is illustrated, in FIG. 5, but another metal mold, corresponding to the metal mold 22 a is also manufactured in a direction in which the recess formed on the inner peripheral surface is covered so as to correspond to the metal mold 22 a. In the mold manufacturing method, when two metal molds are combined, with each other, a mold is obtained the inner peripheral surface of which corresponds to the outer peripheral surface of the cast metal.

In the mold manufacturing method, a wax pattern (a wax mold) is manufactured after the outer metal mold is manufactured (step S16). Hereinafter, this process will be described with reference to FIG. 6. FIG. 6 is a schematic diagram illustrating a wax pattern manufacturing process. In the mold manufacturing method, the core 18 is disposed at a predetermined position of the metal mold 22 a (step S110). Subsequently, a metal mold 22 b corresponding to the metal mold 22 a covers the surface provided with the recess of the metal mold 22 a while surrounding the periphery of the core 18 by the metal molds 22 a and 22 b so that a space 24 is formed by the core 18 and the metal molds 22 a and 22 b. In the mold manufacturing method, wax 28 starts to be injected into the space 24 from a pipe connected to the space 24 as indicated by an arrow 26 (step S112). Here, the wax 28 is a material, for example, a wax, the melting point of which is comparatively low and which is heated and melted at a predetermined temperature or more. In the mold manufacturing method, the wax 28 is charged into the entire area of the space 24 (step S113). Subsequently, a wax pattern 30 in which the wax 28 surrounds the periphery of the core 18 is formed by hardening the wax 28. The wax pattern 30 is formed so that a portion basically formed of the wax 28 is formed, in the same shape as the cast metal to be manufactured. Subsequently, in the cast metal manufacturing method, the wax pattern 30 is separated from the metal molds 22 a and 22 b and a sprue 32 is attached thereto (step S114). The sprue 32 is an opening into which molten metal as melted metal is input during casting. In the mold manufacturing method, the wax pattern 30 which has the core 18 therein and is formed of the wax 28 having the same shape as the cast metal is manufactured as described above.

In the mold manufacturing method, slurry applying (dipping) is performed after the wax pattern 30 is manufactured (step S18). FIG. 7 is a schematic diagram illustrating a configuration of applying slurry to a wax pattern. FIG. 8 is a schematic diagram illustrating an outer mold manufacturing process. In the mold manufacturing method, as illustrated in FIG. 7, the wax pattern 30 is immersed into a storage portion 41 filled with slurry 40 and is extracted so as to be dried (step S19). Thus, a prime layer 101A can be formed on the surface of the wax pattern 30.

Here, the slurry which is applied in step S18 is slurry directly applied to the wax pattern 30. As the slurry 40, slurry obtained by solely dispersing alumina ultrafine particles therein is used. In the slurry 40, it is desirable to use fireproof micro particles, for example, zirconia of about 350 mesh as flour. Further, it is desirable to use polycarboxylic acid as a dispersing agent. Further, it is desirable to add a small amount of a defoaming agent (a silicon material) or a wettability improving agent by, for example, 0.01% to the slurry 40. When the wettability improving agent is added to the slurry, it is possible to improve the adhesion property of the slurry 40 to the wax pattern 30.

In the mold manufacturing method, as illustrated in FIG. 7, slurry applying (dipping) is performed on the wax pattern having the prime layer (the first dry film) 101A by applying and drying the slurry 40 (step S20). Subsequently, as illustrated in FIG. 8, stuccoing is performed in which zircon stucco grains (having an average particle diameter of 0.8 mm) as a stucco material 54 are sprinkled to the surface of the wet slurry (step S21). Subsequently, a layer of the wet slurry in which the stucco material 54 attached to the surface thereof is dried so as to form a first back-up layer (a second dry film) 104-1 on the prime layer (the first dry film) 101A (step S22).

It is determined whether so repeat the process of forming the first back-up layer (the second dry film) 104-1 a plurality of times (for example, n: 6 to 10 times) (step S23). The n-th back-up layer 104-n is laminated a predetermined number of times (n) (step S23; Yes), so that a dried compact 106A is formed as an outer mold with the thickness of a multi-layered back-up layer 105A of, for example, 10 mm.

In the mold manufacturing method, when a dried compact 106A provided with a plurality of layers of the prime layer 101A and the multi-layered back-up layer 105A is obtained, a heat treatment is performed on the dried compact 106A (step S24). Specifically, the wax between the outer mold and the core is removed, and the outer mold and the core are sintered. Hereinafter, this will he described with reference to FIG. 9. FIG. 9 is a schematic diagram illustrating a part of the process of the mold manufacturing method. In the mold manufacturing method, as illustrated in step S130, the dried compact 106A as the outer mold provided with a plurality of layers of the prime layer 101A and the multi-layered back-up layer 105A is heated while being disposed inside an autoclave 60. The autoclave 60 heats the wax pattern 30 inside the dried compact 106A by charging pressurized steam, thereinto. Thus, the wax forming the wax pattern 30 is melted so that melted wax 62 is discharged from a space 64 surrounded by the dried compact 106A.

In the mold manufacturing method, since the melted wax 62 is discharged from the space 64, a mold 72 is manufactured in which the space 64 is formed in an area filled with the wax between the core 18 and the dried compact 106A as the outer mold as illustrated in step S131. Subsequently, in the mold manufacturing method, the mold 72 provided with the space 64 between the core 18 and the dried compact 106A as the outer mold is heated by a combustion furnace 70 as illustrated in step S132. Thus, an unnecessary component or a moisture component included in the dried compact 106A as the outer mold is removed from the mold 72, and the mold is sintered and hardened so that the outer mold 61 is formed. In the cast metal manufacturing method, the mold 72 is manufactured as described above.

Referring to FIGS. 2 to 10, the casting method will be continuously described. FIG. 10 is a schematic diagram illustrating a part of a process of the casting method. In the casting method, the mold is preheated after the mold is manufactured in step S1 (step S2). For example, the mold is disposed in a furnace (a vacuum furnace and a combustion furnace) and is heated in the range of 800° C.-900° C. Due to the preheat treatment, it is possible to suppress the breakage of the mold when molten metal (melted metal) is injected into the mold to manufacture cast metal,

In the casting method, pouring is performed after the mold is preheated (step S3). That is, as illustrated in step S140 of FIG. 10, molten metal 80, that is, a raw material (for example, steel) of melted cast metal is injected from the opening of the mold 72 into a gap between the outer mold 61 and the core 18.

In the casting method, the outer mold 61 is removed after the molten metal 80 poured into the mold 12 is solidified (step S4). That is, when the cast metal 90 is obtained, by solidifying the molten metal inside the mold 72 as illustrated step S41 of FIG. 10, the outer mold 61 is milled so as to be separated as a fragment 61 a from the cast metal 90.

In the casting method, the core removal process is performed after the outer mold 61 is removed from the cast metal 90 (step S5). That is, as illustrated in step S142 of FIG. 10, a core removal process is performed by inserting the cast metal 90 into an autoclave 92. Then, the core 18 inside the cast, metal 90 is melted, and a melted core 94 is discharged from the inside of the cast metal 90. Specifically, the cast metal 90 is dipped in an alkali solution inside the autoclave 92 and is pressurized and depressurized repeatedly so as to discharge the melted core 94 from the cast metal 90.

In the casting method, a finishing treatment is performed after the core removal process is performed (step S6). That is, a finishing treatment is performed on the surface or the inside of the cast metal 90. Further, in the casting method, the quality of the cast metal is checked along with the finishing treatment. Thus, a cast metal 100 can be manufactured as illustrated in step S143 of FIG. 10.

In the casting method of the embodiment, the cast metal is manufactured by manufacturing the mold by the use of the lost-wax casting method using wax as described above. Here, in the mold manufacturing method, the casting method, and the mold, of the embodiment, the outer mold as the outer portion of the mold is formed as a multi-layer structure in which the prime layer (the first dry film as the prime layer) 101A is formed as the inner peripheral surface by using alumina ultrafine particles as slurry and the multi-layered back-up layer 105A is formed on the outside of the prime layer 101A.

Since the coating layer is formed on the surface of the core in the casting method, of the embodiment, the dimensional precision is improved, and hence the durability is improved even at a high casting temperature.

Further, since the high-strength core is provided, the degree of freedom in design (for example, a slow pulling-down speed or the like) of the casting is improved even at the long casting process time.

Furthermore, it is possible to provide a precision-cast product such as a turbine blade which is thin and has good thermal efficiency.

As the precision-cast product according to the invention, for example, a gas turbine stator vane, a gas turbine combustor, a gas turbine split ring, or the like can be exemplified other than the gas turbine blade.

Second Embodiment

Since the embodiment has the same configuration as the precision-casting core of the first embodiment, a description will be made with reference to the drawings (FIGS. 1 and 2) of the first embodiment.

FIG. 1 is a cross-sectional configuration diagram of the precision-casting core.

A precision-casting core according to the invention is obtained by forming a coating layer of two kinds of silica materials having different particle diameters on a surface of a sintered precision-casting core body (hereinafter, referred to as a “core body”) mainly including silica particles.

As illustrated in the upper stage of the cross-sectional view of the core body as the sintered body of FIG. 1, a plurality of holes 18 c is formed in a surface 18 b of a core body 18 a during sintering.

In the invention, as illustrated in the lower stage of FIG. 1, the holes 18 c are sealed by coating the holes 18 c formed in the surface by a coating layer 19 a.

Here, the core body 18 a mainly includes silica particles, for example, melted silica (SiO₂) such as silica sand and silica flour.

The core body is manufactured by a known method in which wax is added to a mixture prepared by mixing silica particles, for example, silica flour (for example, 800 mesh (10 to 20 μm)) and silica sand (for example, 220 mesh (20 to 70 μm)) at the weight ratio of 1:1 and is heated and kneaded so as to obtain a compound.

A core compact is obtained by injection molding the obtained, compound.

Subsequently, the core body 18 a is obtained by performing a decreasing treatment to, for example, 600° C. and a sintering treatment at, for example, 1,200° C.

In the invention, the coating layer 19 a is formed on the surface of the core body 18 a as the obtained sintered body.

The coating layer 19 a includes an alkoxide-silica fume material of an alkoxide material and silica fume.

Here, the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.

In inorganic silica fume, for example, a spherical material having a particle diameter of 0.15 μm is used.

Here, it is desirable that silica fume have a particle diameter of 0.05 to 0.5 μm.

The dispersion ratio of silica fume is set to 5 to 40 wt % and appropriately about 20 wt %.

Silicon ethoxide or silicon butoxide is used as silicon alkoxide, and ethanol or butanol is used as solvent.

Further, when two kinds of alkoxide are mixed, a mixed alkoxide material obtained by mixing silicon alkoxide and aluminum alkoxide is used, and for example, alcohol solvent such as butanol is used as solvent.

When mixed alkoxide is prepared, mixed alkoxide of silicon ethoxide and aluminum isopropoxide is dissolved in a butanol solution.

Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) is mixed at the molar ratio of 2:3 so as to prepare organic mixed alkoxide.

The core sample is immersed into prepared solely alkoxide or mixed alkoxide having silica fume dispersed therein and is pulled up so as to form a silicon layer or a silicon-aluminum alkoxide layer including silica fume on the surface 18 b of the core body 18 a and to precipitate a silicon layer or silicon-aluminum alkoxide component including silica fume even in the hole 18 c of the core surface.

Since solely alkoxide or mixed alkoxide is dissolved in an alcohol solution during immersing, solely alkoxide or mixed alkoxide easily penetrates into the core body, and hence a good coating layer is formed thereon.

Subsequently after the drying process, for example, a heat treatment is performed at 1,000° C. The heat treatment may be performed at, for example, 1,000° C. or less if the surface is provided -with the coating layer 19 a.

After the drying process, alkoxide and silica fume components are precipitated even in the hole 18 c of the surface of the core body 18 a. At this time, a mixed layer is formed by a silica fume layer having a large particle size and a fine and compact alkoxide layer.

Then, since inorganic ceramic is generated in the alkoxide layer due to the heat treatment at 1,000° C., the gap of the silica fume layer having a large particle size is filled by a compact ceramic layer, and hence the adhesion strength among the particles is improved,

In the heat treatment, in the case of mixed alkoxide, the silicon-aluminum alkoxide layer including silica fume changes to inorganic mullite (3Al₂O₃.2SiO₂) having a high melting point due to a reaction. Since the gap of the silica fume layer having a large particle site is filled by a compact mullite layer, it is possible to obtain the core 18 in which the core body 18 a is covered by the coating layer 19 a having the improved adhesion strength among particles.

Here, since the melting point of mullite is 1,900° C. and is higher than the melting point (1,600° C.) of silica, the high casting temperature can be handled.

In this way, according to the invention, since the plurality of holes formed in the surface is sealed, it is possible to prevent a problem in which the core is broken during casting from, the holes as the start points in the related art. Accordingly, the high-temperature strength of the precision-casting core is improved.

Further, since silica fume has a large particle size, heat shrinking is small even at the heat treatment of 1,000° C.

Test Example 2

Hereinafter, a test example for verifying the effect of the invention will be described.

In the test example, a compound was first obtained by adding wax to a mixture prepared by mixing silica flour (800 mesh) and silica sand (220 mesh) at the weight ratio of 1:1 and heating and kneading the mixture. Here, “MCF-200C” (product, name) manufactured by Tatsumori Ltd. was used as silica flour, “RD-120” (product name) manufactured by Tatsumori Ltd. was used as silica sand, and “Cerita Wax F30-75” (product name) manufactured by Paramelt Co., Ltd., was used as wax.

A compact was obtained by injection molding the obtained compound.

As a test sample, a sample having a width of 30 mm, a length of 200 mm, and a thickness of 5 mm was obtained.

Next, a sample for a core body was obtained by performing a decreasing treatment to 600° C. and a sintering treatment at 1,200° C.

Coating Layer 3

Next, silicon ethoxide was dissolved in an ethanol solution. Silica fume (for example, a particle diameter of 0.15 μm; a spherical material) of 20 wt % was mixed with the obtained silicon ethoxide so as to obtain silicon ethoxide slurry mixed with silica fume.

The sample for the core body was immersed into the silicon ethoxide slurry mixed with silica fume and was pulled up so as to form the coating layer 19 a of alkoxide including silica fume on the surface. Subsequently after the drying process, a heat treatment was performed at 1,000° C. so as to form the coating layer 19 a of inorganic silica obtained from silicon ethoxide including silica fume on the core body surface 18 b (Sample 3).

Coating Layer 4

Next, mixed alkoxide of silicon ethoxide and aluminum isopropoxide was dissolved in a butanol solution. Here, mixed alkoxide (silicon ethoxide+aluminum isopropoxide) was mixed at the molar ratio of 2:3 so as to prepare organic mixed alkoxide.

Silica fume (for example, a particle diameter of 0.15 μm; a spherical material) of 20 wt. % was mixed with the obtained mixed alkoxide so as to obtain mixed alkoxide slurry mixed with silica fume.

The sample for the core body was immersed into the mixed alkoxide slurry mixed with silica fume and was pulled up so as to form the coating layer 19 a of mixed alkoxide on the surface thereof. Subsequently after the drying process, a heat treatment was performed at 1,000° C. so as to form the coating layer 19 a on the core body surface 18 b by the mullite including silica fume obtained by the reaction of mixed alkoxide of silicon ethoxide and aluminum isopropoxide (Sample 4).

As a comparative example, a core body without a coating layer was prepared as a comparative sample.

The strength of each of the test samples was measured.

Here, the strength test was performed based on “Bending Strength of Ceramics (1981)” of JIS 1601.

The strength of the comparative sample without the coating layer of the conventional method was 20 MPa, but, to the contrary, the strength of Sample 3 of the silica coating layer of the coating layer 3 for the core body according to the method of the invention was 23 MPa. As a result, in the sample for the core body of the invention, it was acknowledged that the strength was improved by 15%.

Further, the strength of Sample 4 of the silica coating layer of the coating layer 4 for the core body according to the method of the invention was 25 MPa. As a result, in the sample for the core body of the invention, it was acknowledged that the strength was improved by 25%.

According to Sample 4 of the invention, since the high-temperature durability of the core is improved due to the mullite, if is possible to obtain a mold which is not deformed even when she mold is held at a high temperature (for example, 1,550° C.) for a long period of time, for example, when a unidirectional solidified turbine blade is manufactured.

Here, in the casting method using the mold provided with the precision-casting core of the embodiment, the configuration is the same except that the “alkoxide material” as the material of the ceramic slurry 16 used in the method of the first embodiment is changed to the “alkoxide-silica fume material including the alkoxide material and silica fume”, and hence the description thereof will not be presented.

REFERENCE SIGNS LIST

12, 22 a, 22 b METAL HOLD

12 a CONVEX PORTION

14, 26 ARROW

16 CERAMIC SLURRY

18 CORE

18 a CORE BODY

18 b SURFACE

18 c HOLE

19 SLURRY

19 a COATING LAYER

20, 70 COMBUSTION FURNACE

24, 64 SPACE

28 WAX

30 WAX PATTERN

32 SPRUE

40 SLURRY

60, 92 AUTOCLAVE

61 OUTER MOLD 61a FRAGMENT

62 MELTED WAX

72 MOLD

80 MOLTEN METAL

90, 100 CAST METAL

94 MELTED CORE

101A PRIME LAYER 

1. A precision-casting core obtained by forming an alkoxide coating layer including an alkoxide material on a surface of a sintered precision-casting core body mainly including silica particles.
 2. A precision-casting core obtained by forming an alkoxide-silica fume coating layer including an alkoxide material and silica fume of 5 to 40 wt % as the dispersion ratio on a surface of a sintered precision-casting core body mainly including silica particles.
 3. The precision-casting core according to claim 1, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.
 4. A precision-casting mold used to manufacture cast metal comprising: a precision-casting core obtained by forming an alkoxide coating layer including an alkoxide material on a surface of a sintered precision-casting core body mainly including silica particles and having a shape corresponding to a cavity inside the cast metal; and an outer mold corresponding to the shape of the outer peripheral surface of the cast metal.
 5. A precision-casting core manufacturing method comprising: immersing a sintered body of a precision-casting core body mainly including silica particles into an alkoxide material; drying the sintered body; and heating the sintered body so as to form a coating layer on the surface of the precision-casting core body.
 6. A precision-casting core manufacturing method comprising: immersing a sintered body of a precision-casting core body mainly including silica particles into an alkoxide-silica fume material of an alkoxide material and silica fume of 5 to 40 wt % as the dispersion ratio; drying the sintered body; and heating the sintered body so as to form a coating layer on the surface of the precision-casting core body.
 7. The precision-casting core manufacturing method according to claim 5, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.
 8. The precision-casting core according to claim 2, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.
 9. A precision-casting mold used to manufacture cast metal comprising: a precision-casting core obtained by forming an alkoxide-silica fume coating layer including an alkoxide material and silica fume of 5 to 40 wt % as the dispersion ratio on a surface of a sintered precision-casting core body mainly including silica particles and having a shape corresponding to a cavity inside the cast metal; and an outer mold corresponding to the shape of the outer peripheral surface of the cast metal.
 10. The precision-casting core manufacturing method according to claim 6, wherein the alkoxide material includes solely silicon alkoxide or mixed alkoxide of silicon alkoxide and aluminum alkoxide.
 11. The precision-casting core according to claim 3, wherein silicon ethoxide or silicon butoxide is used as the silicon alkoxide.
 12. The precision-casting core according to claim 8, wherein silicon ethoxide or silicon butoxide is used as the silicon alkoxide.
 13. The precision-casting core manufacturing method according to claim 7, wherein silicon ethoxide or silicon butoxide is used as the silicon alkoxide.
 14. The precision-casting core manufacturing method according to claim 10, wherein silicon ethoxide or silicon butoxide is used as the silicon alkoxide. 